Semiconductor devices (in particular, silicon devices) have been increased in the scale of integration and fourfold in every three years by miniaturization (a scaling law called Moore's Law) and reduced in power consumption. In recent years, the gate lengths of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are 20 nm or less. Because of rising costs for lithography processes (equipment prices and mask set prices) and the physical limits of device scaling (operating limits and the variation limits), there has been need for improvement in the performance of highly integrated devices through an approach other than the miniaturization following the scaling law.
Rewritable programmable logic devices, called field programmable gate array (FPGA), which stand midway between gate arrays and standard cell arrays, have been developed in recent years. An arbitrary circuit configuration can be programmed on the FPGA by a user after manufacture of the chip. The FPGA has variable resistance elements inside a multilevel-wiring layer so that the user can make arbitrary electrical connections of wirings. The use of a semiconductor device containing such an FPGA can improve the flexibility of the circuit.
Examples of variable resistance elements include MRAM (Magneto-resistive Random Access Memory), PRAM (Phase Change RAM), and ReRAM (Resistance Random Access Memory). Additionally, there are CBRAM (Conductive Bridging RAM, which is a RAM based on a conductive path using ions in a solid electrolyte) and the like. The operating principles of the memories will be described below.
MRAM makes use of the property of a ferromagnetic material that magnetism generated in the ferromagnetic material by an externally applied magnetic field remains in the ferromagnetic material after removal of the external magnetic field. An MRAM cell uses a structure in which two ferromagnetic layers are stacked with an insulator between the two layers. The magnetization direction of one (fixed layer) of the two ferromagnetic layers is used as a reference magnetization direction and the magnetization direction of the other magnetic layer (free layer) is changed in accordance with data to be stored. Magnetic resistance varies depending on whether the magnetization directions of the two ferromagnetic layers are the same or not. The use of the fact that the value of current flowing through a memory element varies depending on difference in magnetic resistance is used to store data.
Accordingly, to write data, the magnetization direction of the magnetic layer (the free layer) for data storage is set in accordance with the data to be stored to determine the direction of a magnetic field externally applied to the magnetic layer for data storage (the free layer).
“Spin injection magnetization reversal” is being used as a method for writing data on an MRAM. In this method, spin torque applied from a invariable magnetization layer (fixed layer) by direct passage of electrical current through a structure in which two magnetic layers are stacked with an insulating film between the two layers is used to reverse the magnetization direction of free magnetization layer (free layer).
PRAM makes use of the property of a phase-change material that changes to a crystalline state (a lower resistance state) or to an amorphous state (a higher resistance state) in response to an externally applied electrical current, thereby changing its resistance value. A PRAM cell uses a structure having a phase-change layer disposed between two electrodes. The resistivity of the PRAM cell significantly varies depending on the difference between the two phases, crystalline and amorphous phases, of a “variable resistance film” made of the phase-change material. Data is stored in the memory cell by using the fact that current flowing through the memory element varies according to the difference in resistivity between two phases, the crystalline and amorphous phases. A data write determines a current value and a pulse width that causes a phase change from the “low-resistance crystalline state” to the “high-resistance amorphous state” or from the “high-resistance amorphous state” to the “low-resistance crystalline state” in accordance with the data to be stored. This sets the variable resistance film in the “low-resistance crystalline state” or the “high-resistance amorphous state”.
Typical phase-change materials are chalcogenide alloys and a chalcogenide alloy of germanium, antimony and tellurium (Ge2Sb2Te5) is representative. Such a phase-change material (Ge2Sb2Te5) is generally referred to as “GST”.
When the GST in the “low-resistance crystalline state” is heated to a temperature higher than 600° C., the GST loses its crystallinity and, when the GST is subsequently cooled, the GST changes its phase to the “high-resistance amorphous state”. On the other hand, when the GST in the “high-resistance amorphous state” is heated to a temperature higher than or equal to the crystallization temperature and lower than the melting point and the heating state is maintained, “recrystallization” develops and the GST returns to the “low-resistance crystalline state”.
In the PRAM, the phase-change material (GST) in the “low-resistance crystalline state” represents “1” and the state is referred to as the “set state” whereas the phase-change material (GST) in the “high-resistance amorphous state” represents “0” and the state is referred to as the “reset state”.
To rewrite from the “reset state” to the “set state”, i.e. to cause a phase change from the “high-resistance amorphous state” to the “low-resistance crystalline state”, a relatively small current is passed for a long time as a set programing current pulse. Since the phase-change material in the “high-resistance amorphous state” exhibits a high resistance value, even a “small current” can generate Joule heat required for heating the phase change material to the crystallization temperature or higher and the state can be maintained to facilitate “recrystallization”, thereby returning the GST to the “low-resistance crystalline state”.
To rewrite from the “set state” to the “reset state”, i.e. to cause a phase change from the “low-resistance crystalline state” to the “high-resistance amorphous state”, a relatively large current is passed for a short time as a reset programming current pulse. Since the GST in the “low-resistance exhibits a small resistance value, a “large current” is passed to generate Joule heat required for heating to a high temperature higher than 600° C. When the temperature reaches a high temperature exceeding 600° C., the phase change to the “high-resistance amorphous state” is facilitated and whereby the resistance value rapidly rises therefore the current pulse width is set to a short duration to avoid a rapid increase in Joule heat generated.
In PRAM, programming is determined by amplitude regardless of the direction in which current flows and therefore the PRAM can be classified as a bipolar variable resistance element.
ReRAM makes use of the property that a resistance value varies in accordance with whether a conductive path is formed inside a variable resistance film by an externally applied voltage and current to place the ReRAM in the on state or the conductive path formed in the variable resistance film is removed to place the ReRAM in the off state. The ReRAM cell uses a structure that has a variable resistance film disposed between two electrodes. For example, an electric field is applied to form a filament in the variable resistance film made of a metal oxide or a conductive path is formed between the two electrodes to place the ReRAM cell in the on state. On the other hand, then an electric field is applied in the reverse direction to remove the filament or the conductive path formed between the two electrodes to place the ReRAM cell in the off state. The direction of the electric field applied is reversed to make switching between the on and off states where the resistance value between the two electrodes significantly differ. Data is stored by making use of the fact that current flowing through the memory element varies according to the difference in resistance value between the on state and off state. To write data, a voltage value, a current value and a pulse width are chosen such that a transition from the off state to the on state or a transition from the on state to the off state occurs in accordance with data to be stored. This generates or removes a filament for storing data or forms or removes a conductive path.
Non Patent Literature 1 (NPL1) discloses an example of a variable resistance element that is likely to improve the flexibility of a “circuit” used for configuring “memory cells” of ReRAM, among variable resistance elements used for configuration of ReRAM. The variable resistance element uses, metallic ion migration in an ion conductor, “deposition of metal by reduction of metallic ions” and “generation of metallic ions by oxidization of metal” due to electrochemical reactions. Thus the variable resistance element is a nonvolatile switching element that reversibly changes the resistance value between electrodes that sandwich a variable resistance film to make switching. The nonvolatile switching element disclosed in the Non Patent Literature 1 (NPL1) is made up of a “solid electrolyte” made of an ion conductor and a “first electrode” and a “second electrode” each of which is provided on each of the two sides of the “solid electrolyte”. A “first metal” that forms the “first electrode” of the nonvolatile switching element and a “second metal” that forms the “second electrode” differ from each other in standard Gibbs energy of formation ΔG in the process of metal oxidization or metallic ion generation.
In the nonvolatile switching element disclosed in the Non Patent Literature 1 (NPL1), the “first metal” that forms the “first electrode” and the “second metal” that forms the “second electrode” are chosen as follows.
Consider a case where a “bias voltage” which causes a transition from the off state to the on state is applied between the “first electrode” and the “second electrode”. A metal that can be oxidized by electrochemical reaction induced by the “bias voltage” applied at the interface between the “first electrode” and the “solid electrolyte” to generate metallic ions and can supply the metallic ions to the “solid electrolyte” is used as the “first metal” that forms the “first electrode”.
Consider a case where when the “bias voltage” that causes a transition from the on state to the off state is applied between the “first electrode” and the “second electrode”, the “first metal” is deposited on the surface of the “second electrode. The “first metal” deposited on the “second electrode” is oxidized by electrochemical reaction induced by the applied “bias voltage” to generate metallic ions, which dissolve in the “solid electrolyte” as metallic ions. A metal that does not induce the process of metal oxidization and metallic ion generation in some “bias voltage” applied is used as the “second metal” that forms the “second electrode”.
A switching operation in a metal-bridging variable resistance element that achieves an on state and an off state by “formation of a metal bridge structure” and “dissolution of the metal bridge structure” will be described below.
In the process of transition from the off state to the on state (the set process), the second electrode is grounded and a positive voltage is applied to the first electrode. At this point, the metal of the first electrode ionizes at the interface between the first electrode and the solid electrolyte and the metallic ions dissolve in the solid electrolyte. On the second electrode side, on the other hand, metallic ions in the solid electrolyte are deposited as metal in the solid electrolyte by using electrons supplied from the second electrode. The metal deposited in the solid electrolyte forms a metal bridge structure and eventually a metal bridge that interconnects the first electrode and the second electrode is formed. The electrical interconnection between the first electrode and the second electrode by the metal bridge places the switch in the on state.
On the other hand, in the transition process (the reset process) from the on state to the off state, when the second electrode of the switch in the on state is grounded and a negative voltage is applied to the first electrode, the metal of the metal bridge ionizes and the metallic ions dissolve into the solid electrolyte. As the dissolution progresses, a portion of the “metal bridge structure” that forms the metal bridge breaks. The metal bridge interconnecting the first electrode and the second electrode eventually breaks and the electrical connection is disconnected to places the switch in the off state.
Note that as the dissolution of the metal progresses, the “metal bridge structure” that forms the conducting path becomes thinner and the resistance between the first electrode and the second electrode increases. Additionally, the dissolved metallic ions at the interface between the first electrode and the solid electrolyte are reduced and separate out as metal and therefore the concentration of metallic ions contained in the “solid electrolyte” decreases to change the specific permittivity. With this change, electric characteristics, such as capacitance between the electrodes, change before the electrical connection is completely disconnected, and then eventually the electrical connection is disconnected.
When the second electrode of the metal-bridging variable resistance element placed in the off state (reset) is grounded again and a positive voltage is applied again to the first electrode, the transition process from the off state to the on state (the set process) proceeds. In other words, in the metal-bridging variable resistance element, the transition process from the off state to the on state (the set process) and the transition process from the on state to the off state (the reset process) can be reversibly made.
The Non Patent Literature 1 (NPL1) further discloses a configuration of a two-terminal switching element in which two electrodes are disposed with an ion conductor between them for controlling conduction state between the two electrodes, and a switching operation of the switching element.
As embedded memories, on the other hand, a volatile eDRAM (embedded Dynamic Random Access Memory), a nonvolatile flash memory and the like are used. The eDRAM loses stored information when power is removed from it. The flash memory requires a high voltage (5 V or higher). Accordingly, embedded memories have problems, including the problem that they are unsuitable for being embedded in a logic LSI (Large Scale Integrated circuit) which operates at a low voltage (1 V or lower).
(Definition of Polarity of Variable Resistance Element)
Operating characteristics of variable resistance elements that are applicable to the present invention can be classified as unipolar type, which performs resistance varying operation with the level of an applied voltage regardless of the operation principle described above, or bipolar type, which performs resistance varying operation with the level of an applied voltage and voltage polarity.
<Unipolar Variable Resistance Element>
Operating characteristics of a unipolar variable resistance element will be described with reference to FIGS. 15A to 15D. For example, a unipolar variable resistance element including a first electrode, a variable resistance element and a second electrode transitions from an off state (high-resistance state) to an on state (low-resistance state) at a desired set voltage as the threshold voltage when a positive voltage is applied to the first electrode (FIG. 15A). The threshold voltage depends on the thickness, composition, density and the like of a variable resistance film. Then, when a positive voltage is applied again to the first electrode of the variable resistance element in the on state (FIG. 15B), the variable resistance element transitions from the on state to the off state at a desired threshold voltage (reset voltage). As the application of the positive voltage is continued, the set voltage is reached and the variable resistance element transitions again from the off state to the on state.
On the other hand, when a negative voltage is applied to the first electrode (FIG. 15C), the variable resistance element transitions from the off state (high-resistance state) to the on state (low-resistance state) at a desired set voltage as the threshold voltage. Then, when the positive voltage is applied again to the first electrode of the variable resistance element in the on state (FIG. 15D), the variable resistance element transitions from the on state to the off state at a desired threshold voltage (reset voltage).
An element in which the operation in FIGS. 15A-15B and the operation in FIGS. 15C-15D are symmetric and that exhibits resistance variation characteristics that are not dependent on voltage application direction (polarity) but instead is dependent only on the level of voltage is defined as a unipolar variable resistance element.
<Bipolar Variable Resistance Element>
A bipolar solid electrolyte switch element is a switch element that requires voltages of opposite polarities for switching between an off state (high-resistance state) and an on state (low-resistance state). Operating characteristics of a typical bipolar variable resistance element will be described with reference to FIGS. 16A to 16D.
When a positive voltage is applied to a first electrode of a bipolar variable resistance element including the first electrode, a variable resistance element, and a second electrode, for example (FIG. 16A), the bipolar variable resistance element transitions from the off state (high-resistance state) to the on state (low-resistance state) at a desired set voltage as the threshold voltage. Then, when the positive voltage is applied again to the first electrode of the variable resistance element in the on state (FIG. 16B), the variable resistance element exhibits ohmic current-voltage characteristics.
On the other hand, when a negative voltage is applied to the first electrode (FIG. 16C), transitions occurs from the on state (low-resistance state) to the off state (high-resistance state) at a desired set voltage as the threshold voltage. However, when the positive voltage is applied again to the first electrode of the variable resistance element in the on state (FIG. 16D), transition from the on state to the off state does not occur at a voltage higher than or equal to the desired threshold voltage (set voltage).
An element that                transitions from the off state to the on state only when a positive voltage is applied to the first electrode, and        transitions from the on state to the off state only when a negative voltage is applied to the first electrodeas described above is defined as a bipolar variable resistance element.<Definition of Electrode of Bipolar Variable Resistance Element>        
Electrodes used in a bipolar variable resistance element will be defined here. An electrode that transitions from the off state to the on state when a positive voltage is applied as described with reference to FIGS. 16A to 16D is defined as a “first electrode” or an “active electrode”.
<Description of Solid Electrolyte Layer-Based Variable Resistance Element>
As an example of the bipolar variable resistance element described above, the Non Patent Literature 1 (NPL1) discloses a switching element that makes use of metal ion migration in a solid electrolyte layer (a solid in which ions can freely move in response to application of an electric field or the like) and electrochemical reaction. The switching element disclosed in the Non Patent Literature 1 (NPL1) includes three layers: a solid electrolyte layer, a first electrode disposed on one side of the solid electrolyte layer, and a second electrode disposed on the other side of the solid electrolyte layer so that the electrodes are opposed to each other. The first electrode acts as a supplier of metallic ions to the solid electrolyte layer. The second electrode does not supply metallic ions.
An operation of the switching element will be briefly described below.
When the first electrode is grounded and a negative voltage is applied to the second electrode, the metal of the first electrode ionizes and the metallic ions dissolve into the solid electrolyte layer. Metallic ions in the solid electrolyte layer are then deposited in the solid electrolyte layer as metal. The metal deposited in the solid electrolyte layer forms a metal bridge which interconnects the first electrode and the second electrode. The metal bridge electrically interconnects the first electrode and the second electrode to place the switching element in the on state.
On the other hand, when the first electrode of the switching element in the on state is grounded and a positive voltage is applied to the second electrode, a portion of the metal bridge breaks. As a result, the electrical connection between the first electrode and the second electrode is disconnected to place the switching element in the off state. Note that electric characteristics, such as capacitance between the electrodes, change before the electrical connection is completely disconnected, and then eventually the electrical connection is disconnected.
To cause a transition from the off state to the on state, the first electrode is grounded and a negative voltage is applied to the second electrode.
As a switching element implemented by a solid electrolyte layer-based variable resistance element, the Non Patent Literature 1 (NPL1) discloses a configuration and operation of a two-terminal switching element in which first and second electrodes are disposed with a solid electrolyte layer between the electrodes and conduction between the electrodes are controlled.
A switching element implemented by such a solid electrolyte layer-based variable resistance element is smaller in size and on-resistance than semiconductor switches such as MOSFETs. Accordingly, such switching elements are considered to be promising for application to programmable logic devices.
Furthermore, the conduction state (on or off state) of the switching element is maintained after the applied voltage is removed. Accordingly, the switching element may be used as a nonvolatile memory element. For example, a plurality of memory cells each of which includes one selector element such as a transistor and one switching element are arranged in columns and rows. Such an arrangement enables selection of any of the plurality of memory cells using a word line and a bit line. Thus, a nonvolatile memory can be implemented in which the conduction state of the switching element of the selected memory cell can be sensed to read information indicating which of “0” and “1” is stored in the memory cell from the on or off state of the switching element.
Note that the Non Patent Literature 1 (NPL1) discloses a configuration including a first electrode, a second electrode, a variable resistance element connecting to both of the first and second electrodes, and a control electrode connecting to the variable resistance element through a dielectric layer. The dielectric layer is in contact with a side surface of the variable resistance element.
Patent Literature 2 (PTL2) discloses a semiconductor integrated circuit in which a first variable resistance element, a second variable resistance element and a first switching element are connected in series between a first power supply and a second power supply.